Studies of the flow state are important for the planning and design of aircraft and especially of wing profiles. As described in detail by M. Gad-el-Hak in “Flow Control: Passive, Active and Reactive Flow Management”, Cambridge University Press, 2002, study of the flow state onto the wing in operation during cruising flight is desirable especially in the case of technologies for laminar flow control and hybrid laminar flow control. The objective here is to determine and locate the transition of the flow from the laminar state to the turbulent state.
On aircraft wings (or bodies in general around which air flows), a velocity boundary layer forms between the surface and the outer flow, which layer gives rise inter alia to the frictional resistance of the body. The boundary layer is initially laminar and low-resistance. Very small disturbances having a wave character (Tollmien-Schlichting waves) are intensified with increasing propagation in the boundary layer. They cause a transition to a turbulent boundary layer and thus to a greater resistance. The frequency of those disturbing waves depends on the fluid and the flow velocity. In wind tunnel tests they are typically between 10 Hz and 30 kHz. Intensive work is being done worldwide on moving that laminar-to-turbulent change—the transition—on wings and tailplanes to greater wing depths in order to reduce the friction drag thereof.
In that work or similar research and development projects in particular, it is important to obtain an exact determination of the transition in wind tunnel tests and in-flight tests in order in that manner to determine the effectiveness of new techniques and aircraft shapes for reducing friction drag.
In the prior art, arrays of different sensor types are used to measure the transition in wind tunnel tests and in-flight tests. For example, F. Hausmann: “Entwicklung einer Multisensor-Heiβfilmtechnik zur Transitionserkennung im Reiseflug”, Dissertation RWTH Aachen, 2004, describes the use of hot-film sensors, whereas in W. Nitsche, A. Brunn: “Strömungsmesstechnik”, 2nd edition, Springer Verlag, 2006, the use of hot-wire anemometers, PVDF film sensors and microphones is proposed for that purpose.
All of those prior-art flow state sensors share the disadvantage of having a relatively elaborate configuration. A further disadvantage is that all of those sensors provide an analogue sensor signal, which requires laborious amplification of the signal, a high sampling rate and therefore extensive data collection and data evaluation in order for the decision to be made between “laminar” or “non-laminar” at the geometrical location of an individual sensor element. This is described in more detail in I. Peltzer: “Flug-und Windkanalexperimente zur räumlichen Entwicklung von Tollmien-Schlichting-Instabilitäten in einer Flügelgrenzschicht”, Dissertation TU Berlin, 2004.
Hot-wire sensors and hot-film sensors moreover have a high energy consumption and require complex electronics and data evaluation. Particularly sensors with a thermal operating principle are in most cases operated in a closed control loop. For example, a constant temperature is set and, for example, the voltage necessary to maintain the constant temperature is measured and serves as the sensor signal. A comparatively large amount of power is required to operate such sensors.
U.S. Pat. No. 5,272,915 discloses an airflow sensing system in which a hot film sensor is driven by a constant voltage feedback circuit that maintains the voltage across the sensor at a predetermined level. Transitional airflow is distinguished from turbulent airflow by a signal having significant energy in a low-frequency passband from 50-80 Hz. A signal processing circuit drives a three-colour LED display to provide a visual indication of the type of airflow being sensed. A first problem with U.S. Pat. No. 5,272,915 is that a bandpass filter is required in order to sense the presence of energy in the 50-80 Hz passband. A second problem with U.S. Pat. No. 5,272,915 is that it cannot reliably detect a fully turbulent flow state which typically has a significant amount of energy with a frequency greater than 1 kHz.
Pressure sensors, which would also be capable in principle of determining a flow state at a body impinged on by a flow, are sensitive to vibrations and structure-borne sound and to temperature. In addition, they are not sufficiently sensitive for highly dynamic measurements, for example for establishing a transition at high oncoming flow velocities. Robust dynamic pressure sensors, which also have already been used in the field of flow measurement, do not in most cases have the required sensitivity to be used for transition measurements.